The problem of restoring useful vision to a human eye after its cataractus natural lens has been removed has been with us since the introduction of cataract surgery. The solution to this problem has included the use of spectacle lenses, contact lenses, and permanent implantation into the eye of a man-made lens, i.e., an intraocular lens.
Since 1949, when the first implant of an intraocular lens was made, hundreds of thousands of persons have had such implants. Recent advances in cataract surgery have now made the intraocular lens implant procedure a safer and more popular alternative. For example, it is estimated that nearly 40 percent of the people now undergoing cataract surgery select a lens implant, i.e., an intraocular lens, instead of wearing contact lenses or thick cataract-type spectacles.
In addition to advances in surgery which enhance the desirability of intraocular lens implants, there have also been advances in the design of such lenses. Two significant advances in intraocular lens design have been the use of ultrasound eye measurements to determine lens prescriptions and the use of surgical keratometers to reduce visual aberrations (astigmatism) produced by the incision on the cornea. In light of these advances, it is estimated that between about 72 and 82 percent of intraocular lens implant patients achieve 20/40 vision or better.
Although silicate glass was initially considered for use in intraocular lenses, generally such lenses are now made of polymethylmethacrylate. Polymethylmethacrylate is a polymer formed by polymerization of methyl methacrylate monomer.
Both silicate glass and polymethylmethacrylate have many similar properties, e.g., both are inert to body fluids and tissue, are almost perfectly transparent, have constant optical properties, and can be worked mechanically to a high degree of accuracy.
Although polymethylmethacrylate is less hard and, therefore, more easily scratched than glass, it has the major advantage of being lighter in weight (its specific gravity is 1.19 whereas the specific gravity of glass is approximately 3). In addition to the above mentioned properties, polymethylmethacrylate transmits 90 to 92 percent of light, is strong, and can be polished to a smooth finish.
Several methods can be used to form intraocular lenses of polymethylmethacrylate. One such method comprises the steps of coring a lens from a polymethylmethacrylate sheet and machining and polishing the lens to the specifications required for that particular lens prescription. Another method is to injection mold such lenses from polymethylmethacrylate molding material.
As used herein, an "intraocular lens assembly" comprises a lens or lens body as described above formed of glass or polymethylmethacrylate or the like with one or more non-optical "haptic" components or positioners connected to it. Such haptic components are useful for supporting or attaching the lens to the eye and can be formed integrally with the lens or separately and then connected.
In the past, lens assemblies have been provided with haptics having various configurations and properties depending, in part, on the location in the eye into which the lens is to be implanted.
For example, an intraocular lens assembly can be placed in the eye in the location of the removed cataractus natural lens, i.e., in the posterior chamber of the eye. This type of lens assembly is referred to herein as a "posterior chamber lens assembly".
A lens implanted in the posterior chamber of the eye is initially maintained in position by the haptic components of the lens assembly contacting the ciliary body or muscle of the eye. However, in time, the vitreous humor in the eye gradually moves toward the implanted lens assembly and eventually adheres to the back side of the lens opposite the iris. Thus, support for maintaining a posterior chamber lens in its proper position is provided by the haptics and, in addition, by the vitreous humor. Because of the support provided by the vitreous humor, the posterior chamber lens haptics do not have to be rigid structural members and, if fact, can be fairly flexible.
One example of an intraocular lens assembly useful for implanting in the posterior chamber of the eye is disclosed in U.S. Pat. No. 4,159,546 to Shearing. The haptics of this lens are a plurality of non-biodegradable strands which are fixed to the lens body and composed of a flexible material. Only one end of each such strand is secured to the lens, with the opposite end being unsecured.
Another type of intraocular lens assembly is designed to be placed into the eye between the iris and cornea, i.e., into the anterior chamber of the eye. This type of lens assembly is referred to herein as an "anterior chamber lens assembly".
Support for a lens positioned in the anterior chamber is provided almost entirely by the haptics connected to the lens. Thus, the haptics provided in the past for an anterior chamber lens have been generally much less flexible than those provided for posterior chamber lenses. Although such stiff haptics can maintain the lens properly positioned within the anterior chamber of the eye, they have been known to cause trauma to the eye when the eye is rubbed or subjected to other forces.
Additionally, when inflexible haptics of such an anterior lens assembly are squeezed together to fit the assembly into an eye having an internal diameter slightly less than that of the assembly, the lens body is caused to move or vault forward or backward. This can result in the lens body being in front or behind its desired position in the eye. Thus, it is presently preferred that each such lens be sized specifically for the eye into which it is to be implanted.
Because of potential injury to the eye when using lens assemblies with inflexible haptics, lenses designed basically for use in the posterior chamber, i.e., lens assemblies with relatively flexible haptics, have been implanted in the anterior chamber. However, the flexible haptics provided by such lens assemblies in the past have provided less than the desired amount of support to the lens body when implanted in the anterior chamber. Thus, their use in the anterior chamber can result in diopter change. If such a lens moves far enough out of position, it can contact the cornea resulting in permanent injury to the eye.
It is, therefore, desired to provide to the art an intraocular lens assembly that, when positioned in either the posterior or anterior chamber of the eye, will readily maintain itself in proper position and additionally will minimize trauma to the surrounding eye tissue. Further, it is desired to provide a lens assembly that can be used for implantation in eyes having a wide range of sizes.